In the process of xerography, a light image of an original to be copied or printed is typically recorded in the form of a latent electrostatic image upon a photosensitive member, and the electroscopic marking particles, commonly referred to as toner, are developed onto the photosensitive member. The visual toner image is then transferred from the photosensitive member to a sheet of plain paper with subsequent permanent bonding of the image thereto. This bonding of the toner particles onto the paper generally comprises two steps: a first step wherein the toner particles on the paper are partially melted, or otherwise made fluid; and a second step, in which the fluid toner particles are bonded to the paper. In general parlance, these two steps are conceptually combined (since, in many common techniques, the two steps occur substantially simultaneously), and the two steps are together known in the art simply as “fusing.”
In order to fuse the image formed by the toner onto the paper, electrophotographic printers incorporate a device commonly called a fuser. While the fuser may take many forms, heat or combination heat-pressure fusers are currently most common. As one example, one combination heat-pressure fuser includes a heat fusing roll in physical contact with a pressure roll. These rolls cooperate to form a fusing nip through which the copy sheet (the sheet on which the document is finally formed) passes.
Although hot-roll fusing is currently the most common method of fusing in commercially-available electrophotographic printing machines today, numerous other fusing techniques are well known in the art. Fusing by heat alone, by exposing the copy sheet to a heat source, was often used in early plain-paper copying machines. Another popular technique is flash fusing, in which a copy sheet is exposed to a quick and intense flash of radiation which heats only the top surface of the sheet and more specifically mainly the relatively dark areas of toner on the sheet. Finally, another common technique is cold pressure roll fusing, in which no external source of heat is used, and the fusing is carried out by extremely high physical pressure on the sheet. This technique has the advantages of consuming little power, and not requiring any warm-up time, but has the disadvantages of creating images of undesirable gloss and providing a poor fix on solid areas of an image so the toner may come off easily.
Another important technique for fusing is chemical vapor fusing. In this technique, toner on the surface of a copy sheet is made fluid by exposure to a gaseous solvent. Chemical vapor fusing is most often used in situations where high temperatures are to be avoided and thermal fusing would damage the copy sheet. However, many vapor solvents, such as halogenated hydrocarbons, will emit dangerous fumes or become explosive in a high-temperature environment.
No matter which type of fusing is used in an electrophotographic apparatus, fusing is one of the most constraining parameters in the design of any system. Heat-generating fusers consume from 55 to 70 percent of a machine's power during warmup, and require most of the warmup time. Cold roll fusers and flash fusers require large volumes of space in a machine. In hot roll or cold roll fusing, the dwell time of a copy sheet through the fuser is one of the most important limits to the speed of the machine. The fuser is often responsible for most of the environmental problems of a machine, such as noise, heat, and odor. Finally, the fusing step is one of the most crucial in regards to final copy quality. Improper fusing can cause smearing, lack of uniformity of an image, and/or unattractive mottled appearance to an image. For these reasons, designers of copying machines and printing systems require a great flexibility in selecting which type of fuser they wish to use.
When printing on thin, plastic flexible packaging, more difficulties arise in addition to those noted above for printing on paper. For example, xerographic digital printing on thin, flexible film such as plastic may result in unacceptable distortion of the substrates during the roll fusing or radiant fusing process. This is typically due to the excessive substrate temperatures (130-220° C.) reached. In particular, conventional toner materials must typically be heated to high substrate temperatures (130-220° C.) to enable good fusing.
For example, a very commonly used packaging substrate is DuPont Mylar. Digital xerographic printing on Mylar for packaging applications requires a toner fusing step. For either roll nip or radiant fusing of conventional toners, substrate temperatures of 130-220° C. or even higher are typically required to achieve good fusing fix. However, as can be seen from FIG. 1, Mylar substrates undergo significant shrinkage/distortion when subjected to these temperatures in both the machine direction (MD) and the transverse direction (TD). The distortion of Mylar based packaging substrates caused by these fusing temperatures results in an unacceptable packaging film. To reduce this distortion to an acceptable level, fusing temperatures not too much higher than 100° C. are desired.
Therefore, to enable xerographic digital printing without distortion of thin, flexible packaging substrates, there is a need (a) for fusing methods which enable fusing at reduced temperatures below the distortion temperatures of packaging substrates, and (b) for toner materials which fuse at reduced temperatures to enable xerography as a viable technology for digital flexible packaging printing.